Intermediate System to Intermediate System (ISIS) is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. ISIS accomplishes this by determining the best route for packets through a packet-switched network. The protocol was defined in ISO/IEC 10589:2002 as an international standard within the Open Systems Interconnection (OSI) reference design. Though originally an ISO standard, the IETF republished the protocol as an Internet Standard in RFC 1142. IS-IS has been called the de facto standard for large service provider network backbones.
Shortest Path Bridging (SPB) uses ISIS as the control protocol to transfer routing information between devices in an SPB Network acting as a transport network between access networks which may be running different protocols. In an SPB network, the ISIS Link State Database (LSDB) is used to advertise routing information.
When ISIS comes up, there is certain information that needs to be learned (link state information, etc.) in order to build forwarding tables. There can be a lot of churn in all the nodes as the learning of the information happens simultaneously. This is especially true in larger networks or networks where different routing protocols are used within the same network.
ISIS Type Length Values (TLVs) are used to advertise Layer 3 (L3) routes in an SPB Network. It is common practice to deploy SPB Networks where a given route ends up being advertised by more than one SPB Node. In such situations—the best route selection process ends up selecting one of the advertising routers as the L3 Next-Hop Node for each such route. The route then is represented in the Route Table as Lookup (Route Prefix)->(BVLAN, BMAC) where the BVLAN is one of the BVLANs used in the SPB Network and the BMAC is the ISIS system-id of SPB Node selected as the Next-Hop for that Route. Traffic using that route is then forwarded to the selected SPB Node using the (BVLAN, BMAC) based forwarding in the core of the SPB Network.